1. Field of the Invention
The invention is related to the field of nuclear magnetic resonance ("NMR") sensing apparatus, methods and measuring techniques. More specifically, the invention is related to NMR well logging apparatus and methods for NMR sensing within earth formations surrounding a wellbore. The invention also relates to methods for using NMR measurements to determine petrophysical properties of the earth formations surrounding the wellbore.
2. Description of the Related Art
The description of the background of this invention, and the description of the invention itself are approached in the context of well logging because well logging is a well known application of NMR measurement techniques. It is to be explicitly understood that the invention is not limited to the field of well logging.
An apparatus described in U.S. Pat. No. 4,710,713 issued to Taicher et al is typical of NMR instruments used to measure certain petrophysical properties of earth formations from within a wellbore drilled through the earth formations. NMR well logging instruments such as the one disclosed by Taicher et al typically include a magnet for polarizing nuclei in the earth formations surrounding the wellbore along a static magnetic field, and at least one antenna for transmitting radio frequency ("RF") energy pulses into the formations. The RF pulses reorient the spin axes of certain nuclei in the earth formations in a predetermined direction. As the spin axes precessionally rotate and reorient themselves into alignment with the static magnetic field, they emit RF energy which can be detected by the antenna. The magnitude of the RF energy emitted by the precessing nuclei, and the rate at which the magnitude changes, are related to certain petrophysical properties of interest in the earth formations.
There are several principal operating parameters in NMR well logging which should be optimized for efficient operation of an NMR well logging instrument. These parameters include the logging speed (speed of motion of the instrument along the wellbore), the average and the peak power supplied to the instrument and transmitted as RF pulses, and the signal-to-noise ratio ("SNR"). Other parameters of interest include the vertical resolution of the instrument and the radial depth of investigation of the measurements made by the instrument within the formations surrounding the wellbore. The last two of these parameters are primarily determined by the antenna and magnet configurations of the NMR logging instrument. Improvements to these two parameters are the subject of numerous patents and other publications. Providing more flexibility in the instrument's peak power requirements, and limitations on the logging speed necessitated by the physics of NMR measurement have been more difficult to overcome.
A property of NMR measurements made in porous media such as earth formations is that there is typically a significant difference between the longitudinal relaxation time ("T.sub.1 ") distribution and the transverse relaxation time ("T.sub.2 ") distribution of fluids filling the pore spaces of the porous medium. For example, light hydrocarbons and natural gas, as commonly are present in the pore spaces of some earth formations, may have T.sub.1 relaxation times as long as several seconds, while the T.sub.2 relaxation times may be only about 1/1000 that amount. This aspect of NMR well logging is due primarily to the effect of diffusion occurring within static magnetic field amplitude gradients. These amplitude gradients are mainly internal to the pore spaces of the earth formations, and are caused by differences in magnetic susceptibility between the solid portion of the earth formation (referred to as the rock "matrix") and the fluid filling the pore spaces.
In order to perform precise NMR measurements on any medium, including earth formations, the nuclei of the material should be polarized by the static magnetic field for about 5 times the longest T.sub.1 relaxation time of any individual component within the material. This is generally not the case for well logging NMR measurements, since some formation components, as previously explained, may have T.sub.1 relaxation times as long as several seconds (requiring a polarization time of as long as about 30 seconds). This is such a long polarization time as to make impracticable having enough polarization time at commercially acceptable logging speeds. As the instrument moves along the wellbore, the earth formations which are subject to the static magnetic field induced by the instrument are constantly changing. See for example, An Experimental Investigation of Methane in Rock Materials, C. Straley, SPWLA Logging Symposium Transactions, paper AA (1997).
As a result of logging speed considerations, a polarization time of 8 to 10 seconds has become more common for many NMR well logging procedures, including those used for natural gas detection. See for example, Selection of Optimal Acquisition Parameters for MRIL Logs, R. Akkurt et al, The Log Analyst, Vol. 36, No. 6, pp. 43-52 (1996).
Typical NMR well logging measurement procedures include transmission of a series of RF energy pulses in a Carr-Purcell-Meiboom-Gill ("CPMG") pulse sequence. For well logging instruments known in the art, the CPMG pulse sequences are about 0.5 to 1 seconds in total duration, depending on the number of individual pulses and the time span ("TE") between the individual RF pulses. Each series of CPMG pulses can be referred to as a "measurement set".
In the typical NMR well logging procedure only about 5 to 10 percent of the total amount of time in between each NMR measurement set is used for RF power transmission of the CPMG pulse sequence. The remaining 90 to 95 percent of the time is used for polarizing the earth formations along the static magnetic field. Further, more than half of the total amount of time within any of the CPMG sequences actually takes place between individual RF pulses, rather than during actual transmission of RF power. As a result of the small fractional amount of RF transmission time in the typical NMR measurement sequence, the RF power transmitting components in the well logging instrument are used inefficiently on a time basis. In well logging applications this inefficiency can be detrimental to the overall ability to obtain accurate NMR measurements, because the amount of electrical power which can reasonably be supplied to the NMR logging instrument (some of which, of course, is used to generate the RF pulses for the NMR measurements) is limited by the power carrying capability of an electrical cable which is used to move the logging instrument through the wellbore.
Several methods are known in the art for dealing with the problem of non-transmitting time in an NMR measurement set. The first method assumes a known, fixed relationship between T.sub.1 and T.sub.2, as suggested for example, in Processing of Data from an NMR Logging Tool, R. Freedman et al, Society of Petroleum Engineers paper no. 30560 (1995). Based on the assumption of a fixed relationship between T.sub.1 and T.sub.2, the waiting (repolarization) time between individual CPMG measurement sequences is shortened and the measurement results are adjusted using the values of T.sub.2 measured during the CPMG sequences. Disadvantages of this method are described, for example in, Selection of Optimal Acquisition Parameters for MRIL Logs, R. Akkurt et al, The Log Analyst, vol. 36, no. 6, pp. 43-52 (1996). These disadvantages can be summarized as follows. First, the relationship between T.sub.1 and T.sub.2 is not a fixed one, and in fact can vary over a wide range, making any adjustment to the purported T.sub.1 measurement based on the T.sub.2 measurements inaccurate at best. Second, in porous media T.sub.1 and T.sub.2 are distributions rather than single values. It has proven difficult to "adjust" T.sub.1 distributions based on distributions of T.sub.2 values.
Another method known in the art for increasing the power efficiency of an NMR well logging instrument is described, for example in, Improved Log Quality with a Dual-Frequency Pulsed NMR Tool, R. N. Chandler et al, Society of Petroleum Engineers paper no. 28365 (1994). The Chandler et al reference describes using large downhole capacitors to store electrical energy during the waiting (repolarization) time and then using high peak-power during application of the RF pulses in the CPMG sequences to improve the signal-to-noise ratio ("SNR"). There are several disadvantages to the method described in the Chandler et al reference. First, it is very expensive to have large capacitors in a well logging instrument, which must be able to operate at high temperature (generally in excess of 350.degree. F.). Second, using high peak RF power to improve SNR involves complicated and expensive transmitter switching circuits. The switching circuit design problem is only made worse by the requirement that the well logging instrument be able to withstand 350.degree. F. or more. Using high peak power is also not very effective for the purpose of improving SNR because the SNR increases only as the fourth root of the increase in the peak RF pulse power.
Another NMR logging apparatus, known as the Combinable Magnetic Resonance ("CMR") logging tool, is described in U.S. Pat. No. 5,432,446 issued to MacInnis et al. The CMR logging tool includes permanent magnets arranged to induce a magnetic field at two different lateral distances along the wellbore and at two different radial depths of investigation within the earth formation. Each depth of investigation has substantially zero magnetic field amplitude gradient within a predetermined sensitive volume. The objective of apparatus disclosed in the Maclnnis et al '446 patent is to compare the output indications from the first and the second sensitive volumes to determine the effects of borehole fluid "invasion" on the NMR measurements. A drawback to the CMR tool, however, is that both its sensitive volumes are only about 0.8 cm away from the tool surface and extend only to about 2.5 cm radially outward from the tool surface into the earth formation. Measurements made by the CMR tool are subject to large error caused by, among other things, roughness in the wall of the wellbore, by deposits of the solid phase of the drilling mud (called "mudcake") onto the wall of the wellbore in any substantial thickness, and by the fluid content of the formation in the invaded zone.
In NMR well logging measuring techniques, reducing the so-called "dead time" (the time between an initial 90 degree RF pulse and a first one of the 180 degree rephasing pulses in the CPMG sequence) during which no spin-echo measurements are made due to "ringing" of the antenna in the static magnetic field) is important in order to be able to resolve the presence of earth formation components having very short T.sub.2 times. As the dead time is reduced, it becomes necessary in a CPMG pulse sequence to reduce the amount of time ("TE") between individual 180 degree rephasing pulses in the CPMG sequence. Some devices, such as one described in, Measurement of Total NMR Porosity Adds New Value to NMR Logging, R. Freedman et al, SPWLA Logging Symposium Transactions, paper OO (1997), have achieved a time-to-first-echo (and subsequent TE) of as short as 0.2 milliseconds (msec). Since the expected T.sub.2 distribution of typical earth formations extends to one second or more, however, a CPMG measurement sequence of at least 1 sec total length is required to measure the petrophysical properties of typical earth formations. The result of the combination of the need to measure very short and very long T.sub.2 relaxation time components results in an CPMG measurement sequence including 8,000 or more echoes ("echo train") using instruments such as the CMR.
Most petrophysical parameters of interest such as irreducible water saturation, fractional volume of movable ("free") fluid, permeability, etc. are based on only one differentiation between "short" (defined as between 0 and about 33 msec) and "long" (defined as more than about 33 msec) parts of the T.sub.2 distribution. Assuming the CPMG pulse sequence (and resulting "echo train") is about 1 sec in duration, only about 3 percent of the total duration of the echo train is substantially sensitive to components of the earth formation having short T.sub.2 values, as compared to about 97 percent of the echo train being substantially sensitive to components of the earth formation having long T.sub.2 values. The nature of the typical echo train therefore results in stable, precise values for parameters such as the fractional volume of free fluid ("FFI"), but can result in unsatisfactory stability and precision in the values determined for other petrophysical properties such as the irreducible water saturation ("BVI"). See for example, Improved Log Quality with a Dual-Frequency Pulsed NMR Tool, R. N. Chandler et al, Society of Petroleum Engineers paper no. 28365 (1994).
Because the nature of the relationship between petrophysical properties of interest and certain NMR properties is at best uncertain, it is desirable to be able to measure the longitudinal relaxation time T.sub.1 of the earth formations. In NMR well logging measuring techniques, however, T.sub.1 measurement has not proven to be practical using the NMR logging apparatus and techniques known in the art. Even if only low accuracy were required, the most the efficient methods of measuring T.sub.1 would require at least several seconds in between individual measurement sets to enable the nuclei in the earth formations to repolarize along the static magnetic field. Historically, most laboratory and all field measurements of the petrophysical properties of earth formations were limited to measurements of T.sub.1. Based on these results, relationships between the petrophysical properties and the relaxation time T.sub.1 were established. As a matter of practical necessity, however, most commercial applications of NMR measurement to well logging substitute the T.sub.1 relaxation time by measurements of the T.sub.2 relaxation time. In most cases, however, the direct substitution of T.sub.1 by T.sub.2 for petrophysical interpretation cannot be substantiated. The principal reason for the lack of direct ability to substitute T.sub.1 for T.sub.2, is that T.sub.2 is often affected by molecular diffusion within the internal magnetic field gradients present in the pore spaces of earth formations. These internal gradients are caused by differences in magnetic susceptibility, in the presence of the static magnetic field imparted by the NMR instrument, between the solid portion of the earth formations (the rock "matrix") and the fluid in the pore spaces. Smaller size pore spaces generally have larger internal magnetic field gradients than do larger pore spaces, therefore any correlation between pore size and T.sub.1 distribution cannot be directly related to a correlation between pore size and T.sub.2 distribution.
A method for increasing the time efficiency of NMR pulsing sequences is described in U.S. Pat. No. 4,832,037 issued to Granot. The method described in the Granot '037 patent includes applying a static magnetic field to materials to be analyzed, momentarily applying a gradient field to the materials to be analyzed, and applying an RF pulse to an antenna at a first frequency to transversely polarize the nuclei of the material within a specific geometric region. The specific geometric region is the location at which the total magnetic field strength, which is the sum of the static field and the gradient field, corresponds to the Larmor frequency of the polarized nuclei within the specific geometric region. After the gradient field is switched off, the free induction decay ("FID") signal is measured and spectrally analyzed. During a waiting time, generally about equal to T.sub.1, between successive magnetic resonance experiments in the same specific geometric region, additional gradient pulses and RF pulses at different frequencies can be applied to measure the FID signal from different geometric regions within the materials to be analyzed. By measuring the FID signal from within different geometric regions during the waiting time, a plurality of different regions in the materials can be analyzed substantially in the same time span as needed to analyze a single geometric region within the materials. The method in the Granot '037 patent is not useful for well logging, however. First, using gradient pulses as needed for the Granot technique would dramatically increase the power consumption of the well logging instrument. Since the power carrying capacity of the well logging cable is limited, it is not preferred to have additional uses of power in the well logging instrument such as energizing gradient coils. Second, the method in the Granot '037 patent is intended primarily for measurements of the FID signal, rather than measurements of spin echo amplitude decay and T.sub.2 as is more typical of well logging techniques. Using momentary gradient fields superimposed on the static magnetic field would make it difficult to measure spin echo amplitude decay and T.sub.2 since the polarized nuclei in earth formations in any spatial volume would not have an opportunity to return to magnetic equilibrium between successive measurements made according to the technique disclosed in the Granot '037 patent.